EVOH is excellent in transparency, gas barrier property (oxygen barrier property and the like), aroma retaining property, solvent resistance, oil resistance, mechanical strength and the like, and are typically formed into films, sheets, bottles and the like, which are widely used for packaging materials such as food packaging materials, medical drug packaging materials, industrial chemical packaging materials, agricultural chemical packaging materials.
Such an EVOH is typically prepared by preparing an ethylene-vinyl ester copolymer through copolymerization of ethylene and a fatty acid vinyl ester such as vinyl acetate, and saponifying the ethylene-vinyl ester copolymer in an alcohol solvent such as methanol in the presence of an alkali catalyst under higher-temperature higher-pressure conditions.
It is a conventional practice to convert the EVOH alcohol solution prepared under the higher-temperature higher-pressure conditions through the saponification into an EVOH water/alcohol mixture solution which is stable at an ordinary pressure, extrude the EVOH water/alcohol mixture solution in a coagulation bath mainly containing water at a lower temperature for solidification in the form of strands, cut the strands for pelletization, and dry the resulting pellets to provide a product.
The EVOH pellets provided by this production process are porous. A widely used method for incorporating an additive, particularly a thermal stabilizer such as a carboxylic compound, a boron compound and a phosphoric compound, in the EVOH pellets is such that the pellets are immersed in an aqueous solution of the thermal stabilizer to be thereby impregnated with the aqueous solution, and dried.
In this EVOH preparation process, however, the alcohol is liable to run into the coagulation bath when the EVOH water/alcohol solution is extruded into the strands in the coagulation bath. The alcohol is evaporated in air, thereby impairing a working environment.
To solve this problem, there has been proposed an EVOH pellet production process (see, for example, Patent Document 1) which includes the steps of feeding an EVOH alcohol solution into an apparatus and bringing the EVOH alcohol solution into contact with water in the apparatus to replace the alcohol with water to provide a hydrous EVOH composition (Step 1), pelletizing the hydrous EVOH composition (Step 2), drying the resulting pellets (Step 3), melt-kneading the pellets having a reduced water content by means of an extruder (Step 4), and cutting an EVOH extruded from the extruder into pellets (Step 5).    Patent Document 1: W02004/009313 (US2006/108703A1)
The EVOH pellet production process disclosed in Patent Document 1 is excellent in that the alcohol removed from the EVOH alcohol solution can be recovered without exposure to the ambient atmosphere and, therefore, the working environment is not impaired.
It is a common practice to blend the thermal stabilizer such as the carboxylic compound, the boron compound and/or the phosphoric compound with the EVOH to improve the thermal stability during melt-forming. As exemplary methods for incorporating the thermal stabilizer in the EVOH, Patent Document 1 discloses a method in which the pellets provided in Step 2 are immersed in an aqueous solution of the thermal stabilizer to incorporate the thermal stabilizer in the pellets, and a method in which the thermal stabilizer is added to the EVOH in the extruder during the melt-kneading in Step 4. However, the pelletization in Step 2 of the process according to the invention disclosed in Patent Document 1 is not achieved by a coagulation method, so that the resulting pellets are not porous. This makes it impossible to sufficiently impregnate the pellets with the thermal stabilizer, often failing to sufficiently provide the effect of the thermal stabilizer.
Where the thermal stabilizer is blended in the EVOH during the melt-kneading in Step 5, the thermal stabilizer is not sufficiently distributed throughout the EVOH at the initial stage of the melt-kneading. Therefore, the pellets are liable to be slightly colored due to heat history at the initial stage.